Electric coupling circuits



` Mar ch-25, 1930. H. wHlTTLE 1,752,046

ELECTRIC coUPLING CIRCUITS l Filed April 6, 1927 ffy u I' A ar/vey Patented 25, 1930 LABORATORIES, INCORPORATED, vO14" N YORK A.`IY'ER SEY.ASSIGNB' T0 BELL TELEPHONE W YORK, N. 4Y., 'A CORPORATION 0F NEW ELECTRIC' COUPLING CIRCUITS Application led April 6, 1927. Serial No. 181,530.

This invention relates to transformer circuits and particularly to input transformer circuits for v electric discharge devices reuired to transmit waves of a wide range of equency. v

Transformers are ordinarily employedI in transmission systems for matching impedances between individual parts to obtain efflcient and uniform transmission of the desired waves.. With an electric discharge amplifier the output current is proportional to the input voltage, and uniform power throughout the frequency rangewill be delivered provided the input voltage is constant. The primary requirement for an input transformer, ltherefore, is that the ratio between the supply voltage and the input voltage be kept constant throughout the transmitted frequency range. Generally, it is required to match the impedancesi on the primary side of the transformer, but this is not the controlling factor. Heretofore, it has been found that the transmission is improved by resonating the leaka e inductance of the transformer with the e ective input capacities of the discharge device. Also, the requirement of impedance match, is attained by the use of a resistance element,`having a resistance approximately `equal to the impedance of the input circuit, connected in shunt to the primary Winding of the transformer. However, this arrangement gives a uniform transmission characteristic only for one particular transformer ratio, while other ratios give anon-uniform characteristic, so

that it is notalways possibleto use the most advantageous transformer ratio. Similarly, a resistance in shunt to the secondary winding of the transformer tends to give a falling characteristic. v l

In a specific preferred embodiment this invention comprises an 'input transformer arrangement foran electric discharge amplier, in which, the leakage reactance` of the input transformer resonates with the coins bined distributed capacity of the secondary winding and the input capacity of the electric discharge amplifier at a frequency near the upper limiting frequencyfof la wide range of frequencies to be transmitted, and a resistance element is connected in shunt to each of the windings of the transformer. These elements are so proportioned with respectto one another and the transformer ratlo that the impedance looking into they primary of thetransformer with the resistance in shunt therewith is matched' to the impedance of the transmitting'circuit at frequencies in the lower portion of the range of frequencies to be transmitted. The resistances are further so proportioned with respect to the leaka e reactance at the resonant frequencythat t e ratio between the inputvoltage and the secondary voltage of the transformer is the same fat the resonant frequency as at the frequencies at which the primary impedances are matched. v This arrangement has been found to maintain the voltage ratio substantially constant throughout the intermediate frequency range and 1s very flexible, making ossible the choice of the preferable transfbrmer ratio.

While this invention is particularly described as applied to an input transformer for an electric discharge amplifier it may be found useful for use with other circuits of capacity reactance. matching is not the primary goal it has been found that the arrangement of this invention gives a substantial impedance match through- Although impedance out the larger portion of the frequency range of thewaves to be transmitted.

lThis invention will be better understood by `reference to the following detailed description taken in connection with the accompanying drawing in which: Fig. 1 represents in schematic form a repeater circuit in accordance with this inven- 4tlou comprising an input transformer serving as a coupling between a transmission line and avacuum tube amplifier;

'y values of the resistances connected in shunt to the windings of the transformer may be determined for any given condition; A

Fig. 5 shows the characteristic curves rep-v resenting the voltage amplification, or gain, of the transformer circuit working into the vacuum tube and covering a range of frequencies from 16 cycles to lapproximately 31,000 cycles, for three different conditions, namely, when 'a resistance is connected in shunt to the primary winding only, when the resistance is connected in shunt to the sccondary winding only, and when two resistances are connected in shunt to the primary and secondary windings respectively; and

Fig. 6 represents a transmission system employing the amplifier circuit of Fig. 1 in connection with an output transformer.

The arrangements shown in the several figures of the drawing are particularly adaptable to signaling systems in which uniform transmission is required over a wide band of frequencies ranging, for example, from about 20 cycles to 31,000 cycles per second,and these arrlangements will now be described in detai Referring to Fig. 1, there is shown an input transformer 12, employed for coupling an amplifying vacuum tube 13 to a line 14 which is connected at its opposite end to a source 15 of waves of a wide frequency range. Connected in shunt respectively to the primary and secondary windings ofthe transformer are resistance elements 16`and 17 which have values so proportioned with respect to each other and tothe leakage reac'tance of the transformeras to substantially match the im-y pedance of the transformer with the impedance ofthe line throughout a large portion of a wide band of frequencies ranging from about 20 cycles to approximately 31,000 cycles f 'per second. The transformer is so designed that the` leakage inductance resonates withl the combined distributed capacity of the secondary winding and the input capacity ofthe amplif ing vacuum tube at substantially the upper iimiting frequency of the above mentioned band. This combined capacity is represented'in the circuit of Fig. 1 by the dotted condenser across the input circuit of tube 13. lThe impedances 0f the primary circuit of the transformer are matched to the Iimpedance of the line at the lower limiting 'frequency of the band. This arrangement gives a constant yalent of that shown in Fig. 2.

ratio between the input voltage and the secondary voltage of the transformer throughout the entire frequency band. While heretofore it has been possible to obtain uniform transmission for a fairly wide range of frequencies the choice of transformer ratios was generally very limited, with Kthi'sinvention values which will keep the value of the ratio 1' (TZ between 0 and 2, as wlll hereinafter be described, 1 beingthe impedance ratio of the transformer, Z1, the input impedance and wL the leakage reactanceatl the resonant frequency# In connection with the coupling c1rcu1t ywhich has'just been described, design formulee were developed which show the relation between the shunting resistances and the `leakage reactance at the resonant frequency,

the transformer coil ratio, the transmitting end impedance, and the values of .the shunt resistances 16 and 17 required to. obtainv a substantially flat transmission characteris tic for the combination.v For the .purpose of analysis the essential parts of the circuit of Fig. 1 are shown in a simplified schematic 4the transformer ratio is only limited to those v form in Fig. 2 in which thetransformer 12 e is replaced` by the well known equivalent T network of unity ratio. In this network the inductances of the arms are designated in terms of leakage inductance of which L in dicates the total leakage in both primary and secondary windings of the transformer and icl indicates the mutual inductance. 'Ihe resistance elements 16 and 17 are designated R1 and R2 respectively for the purpose of analysis and in assuming the transformer to be of unity ratio the electrical impedances on the primary side of the transformer are multipled by 7' which represents the impedance ratio of the transformer. In this way the transmitting or input impedance Z1 becomes rZl, the resistance R1 becomes 7R and the transmitting voltage e becomes ei/r.

In this discussion the mutualinductance cZ (of which lc is the coupling factor of the transformer and Z thehigh side inductance) may'be disregarded at most of the frequen- 'cies under consideration since the value of cZ is high as compared with L.

Neglecting the mutual inductance CZ the schematic circuit shown in Fig. 3 is the equiv- The following formulae have been developed for the purpose of determining the values to be assigned to the resistances R1v and R2 inorder that the transmissionchar- .l

acteristic of the coupling circuit will be sub- .stantially fiat over the entire range. In these formulae C denotes the capacity between the `input electrodes of the amplifying vacuum tube 13 plus the distributed capacity of the transformer, V2 represents the multiplied by the frequency. v (2)'l V2 vg+ jwL (1,) V, Ham/( +1200) 1 (3) I2= y Formula (8) is obtained from quation- (7) by limiting w to the resonant frequency, or w2LC=1. Solving for the absolute value of 40 Now at low frequencies where wL is small and wlais large as compared to the value ofR,

(10) Z2 =I1`lz two resistances in parallel.

45 Therefore by makin i Z2=r`Z`1 to properly match the impedances etween'the transmit# ting end and the receiving end.

TRlRg l 60 .(11) l fzflml (12) Then R1+1)=y2%whichcan besubstitutedinEquation (9). y y

Equation (13) might be solved for R2 di-- p rectly but as R2 appears in all powers up to 60 R2* the solution would be very complicated. It is therefore preferable to-solve for rZl in terms of R2 giving an implicit solution.

Since at low frequencies e.' =2', as half the 3 65 transmitting voltage is consumed in the transmitting impedance when the impedances a filter.

rZ1 (13) lfor E f From the fact that,

are matched, this ratio should also equal 2 TRIR, e at the resonant frequency in order to main- TZV-jm tain the voltage ratio constant up to the resi or i onant frequency after which the transmissionv Z1 ,Z1 v falls rapidly corresponding to the cut-off of -+-*=1 TR1 J? 01' Assuming -T7-=2 and lsolvlng Equation rZ1 Z1 Z1 s 1 TR1 R1 R2 l it follows that the ordinate distance between 4M l' ML2' 2 2 2 @it @Q R22) 4 w0R2+R22 1- R22 4- 15 ce R,

Multiplying through, simplifying and substitutingwL for T R2 2; (9L 2- l ai) n -1 By calculating if? with various values of and than dividing,

` when wL=rZ1 and is infinite when rZ1=0 or when if is eL is infinite. To obtain L zero, it is assumed that is very small onpared to 1,-jand substitute in Equation v R rZi p z Iig wL or? when gi is large compared to 1 Since the tworesistances R1 and R2, which are effectively in parallel when viewed from the low side of the transformer, must equal l Z rZl, then must be a minimum when rZI=wL as indicated in Fig. 4. x

2 2 [s202152 ige i] the curve and unity represents so if it is assumed that and increasing values of are plotted downlf ward, the same curve can be used for obtain- L ing R1 as for obtaining Rh'lz; is then equal to unity at the ends of the curve and is minimum when wL=rZ1 when it approximately .Then R,

in which it is assumedlthat Z1=600 ohms,"v r=44 and l co0 ohms at 31,000 cycles then rZl =26,400

and

From the curve 4= 51,700 ohms In designing a transformer according-to this invention the transformerlow side impedance can be assumed as a shunt impedance' between two equal resistances equalto the transmitter impedance, which is-assumed to be resistive,'when considering the lowest frequency to be transmitted. This impedance can be made such a value that the transmission' loss caused by it at the lowest frequency is less than a certain amount. The required ratio and the coupling factor of the design used are then determined in such a way `that the leakage inductance considered from the v pedances at the primary side of the trans-i transformer is the approximtaely fmer 12 and the output tem re uired to transmit waves of frequencies approximatel `transfzr'mer may be of the shell type, for exhigh transmission at lthe lower frequencies,

able sacrice is made 1n the step-up voltage of amples.

high side will resonate with the tube and the transformer. A-shell type transformer transformer coil capacities at or above the upwith pure resistances connected respectiyely per limiting frequency. From the ratio of across the primary and secondary windings ythe transmitting mpedancemultiplied by the thereof and proportioned in the manner as impedance ratio, to the hi h side leakage rehereinbefore stated, may be adapted to conactance at the resonant requency, the abvnect a 600 ohm line to the input of a vacuum l the design chart i shown in Fig. .4, the ordinates of the curveA the transmitted signals. The shell'type core is giving the relation of the shunting resistpreferably made `up of E and I shaped lamiances R1 or R2 to the transmitting end imnations of a nickel-iron alloy having 78.5% pedance and coil impedance ratio. Other de nickel, generally known as 78.5% permalloy. sign charts ofcourse can be derived if any The laminatioiis are staggered in such amanthis is the most usual condition in telephone mutual inductance. practice.

In Fig. 5 are shown transmission characteristic curves obtaine'dby tests and representing in each case the relation between the voltage amplification. or ratio of a particular Mounted upon the core are primary and secondary windings which are also interleaved for the purpose of cuttingY down the leakage between them. It was found necessary to employ this type ofconstruction to obtain the pure resistance element is coning such a wide band of frequencies.

nected in shunt to the primary winding only; The output transformer may be of the shell in another the resistance element is connected typev but preferably should be of the toroidal in shunt to the secondary winding only; and type, the core being a solid ring of 78.5% still, in another two resistance elements are permalloy and have mounted thereon priconnected in shunt to the primary and secmary and secondary windings interleaved, ondary windings respectively as shown in these features being provided foithe same Fig. 1. -In the first two cases, these resistances purpose as stated in connection with the inwere given Asuch values as to' match the imput transformer. The shell type of output same as the ,input transformer to that ofthe line. For the case of the former described above except that no resh-unt across the primary winding, the charsistive shunts are acteristic indicatedby A rises to a peak at mary and secondary windings andthe posithe high frequencies while for the shunt l across thesecondar7 causesthe characteristic circuit is reversed so that it acts as a step' indicated by B droops in the same range.v downftransformer. The output transformer The arrangement of this invention as indimay have in addition av third winding (not cated by curve D gives a substantially conshown) energized from battery 21 in series stant voltage ratio even at the high frequenwith a resistance (not shown) to produce suf- 'cies of a band ranging from about 20 cycles to cient fiux to neutralize the effect of the flux 31,000 cycles per second. set-up by the direct current in the primary This voltage is obtained for various coil rawinding. l

tios and it is to be noted that the gain in trans- The toroidal output transformers are capamission units is 'constant within oneT. U. for ble of transmitting a band of frequencies a` range of frequencies from 20 cycles to which is still wider than the band transmit- 31,000 cycles. i l ted by 'the in ut transformers described, the n In Fig.',6 is illustrated a transmission sysyoutput transfnrmer frequency vfrom`5 cycles to 35,000 cycles he output transformers do not from a out 20 cycles to 31,000 cyclesA per secper second. Q comprsesa transmitting de'- require resistances connected across the priond andg'which windings thereof, ofthe band, inasmuch as there is no distora vacuum tube amplifier 13, output transfortion at this point for the reason that the cou- The 'input transforpled circuits have impedances which are practransformer 19 are tically independent of frequency, the matchtypes. I 'The input ing of impedances being obtained by the particular ratio used..-

It is to b e understood that the numerical in. order to obtain a values that have been given in the foregoing primary impedanceA to improve the specification,` for the frequency limits of the but band orfor other' quant1ties, are not to be impedance a considertaken as limiting but are cited by way of exprefe'rably of different ample, and may have 'a lower coil ratio than hereinbefore assumed with a higher primary bandv ranging ondary circuit having a capacity reactance to transmlt a' wide band of frequencies, comprisinga transformer having primary and secondary windings andjleakage inductance, said leakage inductance being of such avalue as to resonate with said capacity' reactance near the upper limit of said band, and resistances in shunt, respectively, to said primary and secondary windings, and proportioned lwith respect to each other so as to give said coupling circuit a substantially flat transmission characteristic over said band.

2. A circuit for coupling aline to the input of a secondary circuit having capacity reactance, to transmit a wide band of frequencies, comprlsing a transformer, having primary and secondary windings and leakage inductance, said leakage inductance being of such value as to resonate with said capacity reactance at a frequency near the upper limit of said band, and resistances in shunt, respectively, to said primary and secondary windings and proportioned with respect to each other and said leakage inductance so as to give said coupling circuit a substantially flat' transmission characteristic over said band.

3. A circuit for coupling a line to a secondary circuit having a capacity reactance to transmit a wide band of frequencies, comprising a transformer having primary and secondary windings and leakage inductance, said leakage inductance being of such a value as to resonate with said capacity reactance near the upper limit of said band, and resistances in shunt, respectively, to said primary and secondaryv windings and proportioned with respect to each other and said leakage inductance so las to give said coupling circuit a substantially-flat transmission characteristic over vsaid band `and to substantially match the impedance of said .line to the impedance of the primary circ'uit of said transformer over the greater portion of said band.

4. A circuit for coupling a line to a secondary circuit having a capacity reactance to P transmit a wide band of frequencies, and comprising a transformer having primary and secondary windings, said transformer having series inductive reactance and distributed shunt capacity, the total effective shunt capacity impedance. including the shunt capacity reactance of said secondary circuit and the distributed capacity reactance of said transformer windings, and the-total effective series inductive reactance being proportioned so as to be equal at the highest frequency of the bandto be transmitted, and resistances respectively in shunt tosaid primary and secondary windings and lso proportioned with respect to each other-and said togli@ fective series inductive reactance as t0 give a transmission characteristic having a maxlwide range of frequencies, a line receiving waves therefrom, an electric discharge device, an input transformer having primary and secondary windings for coupling said line to said device, the leakage inductance of said transformer being such as to resonate with the combined distributed capacity of said secondary winding yand the input capacity of said device at a frequency near the upper limiting frequency of said range, and a resistance element in shunt to each of said windings,` the resistances of said elements being so proportioned with respect to each other and the leakage reactance at the resonant freuency that the impedance into the primary winding and associated shunt resistance is substantially equal to theline impedance at frequencies in the lower portion of said range and that the ratio of the voltage of said source to the secondary voltage of said transformer is substantially constant throughout said range.

6. A combination according to the preceding claim in which the relation of the resistances of said elements is substantially represented by the equation:

t R1 R2 in which R1 and R2 are the resistances of the elements in shunt to the primary and secondary windings respectively, Z1 is the impedance of the line, and 1 1s the impedance ratio of the transformer. i 7. In combination, a source of waves of a wide range of frequencies, a line receiving waves therefrom, an electric discharge device, an input transformer having primary and secondary windings for coupling said line to said device, the leakage inductance of said transformer being such as to resonate with the combined distributed capacity of said secondary windin and the input capacity of said device at a equency near the uper limiting frequency o said ran e,. and a resistance element in shunt to eac of said windings, the relation o f the resistance of said elements to each other and to the other elements of the circuit being substantially represented by the following equations:

c292+ (til-1"" in which R1` and R2 are the values of the reof the transformer, and wL is the leakage re actance at the resonant fre uency.

8. In combination, a line a apted for transmitting waves of a wide range of frequencies, an electric discharge device, an input transformer having primary and secondary windings for coupling said line to said device, the leakage inductance of said transformer being o such value as to resonate with the comm bined distributed capacity of the windings and the input capacity of said device at a frequency near the upper limiting frequency of said range, and a resistance 'element in shunt to each of said windings and of such relative value as to damp the resonance effect at said frequency, whereby the voltage amplification of said transformer is made substantially constant throughout said range.

' In witness whereof, I hereunto subscribe 2.0- my name this 5th day of April, A. D., 1927.

HORACE WHITTLE. 

